Electrovalve diodes

[CitroJim]

Would be interesting to see a scope trace of the current flowing through a good working EV...
- during the 0.5sec 12v pull in
- during PWM hold in
- at switch off
Is that something that you could do on the bench, Jim?

Best I can do right now Chris is this:



I took that a few years ago now when I was chasing a problem of fast regulator tick in soft mode on one of my V6 Xantias...

Duff diodes show up as significant ringing and overshoot at the squarewave transition points...

[xantos]

viewtopic.php?f=3&t=44691&hilit=electrovalve

viewtopic.php?f=3&t=27963&p=191456&hili ... pe#p191456

Here is also some good info...

I can't believe how many times we chewed this problem...

[white exec]

Just read that 5-page 2009 thread (Jim's fast regulator tick), and details of the electrovalve leakage.

Breakthrough was the realisation of the separate functiions of the two springs. What is surprising is the sheer strength of the larger spring, which the solenoid action has to compress. No wonder there is a loud click when an EV is de-energised.

Not sure I understand how hydraulic pressure would help keep the shuttle open, as although the cylindrical core is something of a piston-fit in the aluminium bore, it is drilled through, and therefore pressure at each end of it would be equal, and no fluid through-flow when it is magnetically pulled in.

Just remain surprised at the toughness of the large spring . . . presumably to combat incoming HP pressure, and ensure valve the shuts for Firm mode.

As you say, a topic much chewed over. Should never have cut that solenoid open!!

[CitroJim]

Should never have cut that solenoid open!!

Perhaps but it's been very educational to see the insides Chris... I now know why they can survive being continuously run on direct ignition-switched 12V - a terrible bodge I've seen on many XMs to force permanent soft mode

Of all the XMs I've seen like this, a return to normal wiring, an ELIT diagnostic and a quick diode mod has restored normal hydractive performance...

[xantos]

Should never have cut that solenoid open!!

Oh come on don't be sorry... Now that you dug up the bone let's have a feast!

[white exec]

Yes, let's. I was just being tactful, but got rumbled.

[demag]

I did the old pcb mod on my Xm. The back one was working but not the front when I bought it. But once I did the mod everything was fine.

[white exec]

Good to get these reports of failed/failing EVs returning to normal operation when external diodes are fitted. Seems to confirm the sense of fitting them anyway, as a preventative measure.

Did a bit of background reading on the need for a working flyback diode when a PWM supply (such as our suspension ECUs) is used to supply a solenoid.

One PWM device that is used to drive solenoids (inductive loads) is Texas Instruments' DRV102. Although its output stage is not the same, nor as high-power, as the (mosfet) VN05N our ECUs use, it does detail the importance of using a diode to ensure proper working of the solenoid under pulsed conditions:
http://www.tij.co.jp/jp/lit/ds/sbvs009b/sbvs009b.pdf

from pp.8 and 9...

BASIC OPERATION
The load (solenoid, valve, etc.) is connected between the
output (pin 6) and ground. For an inductive load, an external
flyback diode (D1 in Figure 1a) across the output is required.
The diode serves to maintain the hold force during PWM
operation. Depending on the application, the flyback diode
should be placed near the DRV102 or close to the solenoid
(see “Flyback Diode” text). The device’s internal clamp
diode, connected between the output and ground, should not
be used to carry load current. When driving inductive loads,
an additional diode in series with the out pin, D2, is required
(see “Series Diode” text).

FLYBACK DIODE LOCATION
Physical location of the flyback diode may affect electromagnetic
radiation. With most solenoid loads, inductance is
large enough that load current is virtually constant during
PWM operation. When the switching transistor is off, load
current flows though the flyback diode. If the flyback diode
is located near the DRV102 (Figure 2a), the current flowing
in long lines to the load is virtually constant. If the flyback
diode is, instead, located directly across the load (Figure 2b),
pulses of current must flow from the DRV102 to the distant
load. While theory seems to favor placing the diode at the
DRV102 output (constant current in the long lines), individual
situations may defy logic; if one location seems to
create noise problems, try the other.

SERIES DIODE FOR INDUCTIVE LOADS
An additional bias diode, located in series with the output, is
required when driving inductive loads. Any silicon diode,
such as the 1N4002, appropriately rated for current will
work. The diode biases the emitter of the internal power
device such that it can be fully shut off during the “off”
portion of the PWM cycle. Note that the voltage at the load
drops below ground due to the flyback diode. If it is not used,
apparent leakage current can rise to hundreds of milliamps,
resulting in unpredictable operation and thermal shutdown.

I thought this made interesting reading, and confirms what Simon, Mike, Jim and others have written about pulsed working of the solenoids. That last paragraph (about the series diode) is interesting, too.

[CitroJim]

Chris, that is indeed interesting reading Thanks for posting that!

[Mandrake]

After reading that to me it sounds like the flyback diode should have always been in the ECU housing not in the electrovalve! In which case fitting the replacement diodes at the ECU end of the cable would seem to be the better course of action both from a physical point of view and interference suppression.

Who would have thought Citroen could make a design mistake in the electrical systems of their cars...

[CitroJim]

Who would have thought Citroen could make a design mistake in the electrical systems of their cars...

That comment made me howl Simon

Mind you, had Citroen done it more correctly then we may have lost the facility to 'listen' to the electrovalves on the LW band of the car radio